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Abstract

An integrated heterodyne optical phase-locked loop was designed and demonstrated with an indium phosphide based photonic integrated circuit and commercial off-the-shelf electronic components. As an input reference, a stable microresonator-based optical frequency comb with a 50-dB span of 25 nm (~3 THz) around 1550 nm, having a spacing of ~26 GHz, was used. A widely-tunable on-chip sampled-grating distributed-Bragg-reflector laser is offset locked across multiple comb lines. An arbitrary frequency synthesis between the comb lines is demonstrated by tuning the RF offset source, and better than 100Hz tuning resolution with ± 5 Hz accuracy is obtained. Frequency switching of the on-chip laser to a point more than two dozen comb lines away (~5.6 nm) and simultaneous locking to the corresponding nearest comb line is also achieved in a time ~200 ns. A low residual phase noise of the optical phase-locking system is successfully achieved, as experimentally verified by the value of −80 dBc/Hz at an offset of as low as 200 Hz.

Fig. 4 (a) Schematic diagram of the set-up of the optical frequency comb (OFC) in a MgF2 crystalline whispering gallery mode (WGM) resonator. The distributed feedback (DFB) laser pumps the resonator using an evanescent wave prism coupler. The generated frequency comb leaves the resonator through prism couplers. The light exiting one of the prism couplers was sent to a fast RF photodiode and optical output was obtained from the other coupler, (b) optical microscope image of the MgF2 crystal forming optical WGM resonators, (c) packaged OFC unit with green fiber pigtail, (d) optical spectrum of a stabilized Kerr frequency combs (left) generated in the unit (right). The comb spans 3 THz defined as the width where the intensity ≥ 50 dBm (red dotted line) and has a line spacing of 25.7 GHz, yielding more than 115 lines. The optical output comb power exiting the fiber (greenjacketed) is 100 µW obtained after subtracting from the pump laser power, meaning only ~0.5 µW per comb line is achieved in the wavelength range of 1535 nm-1575 nm. The horizontal (green) dashed line denotes the 0.5 μW per comb line power level. (e) Clearly observed lines of a multi-soliton Kerr frequency comb with a spacing of 0.2 nm.

Fig. 6 (a) Optical spectrum when SG-DBR laser and comb are phase locked with a frequency difference of 11 GHz. The locking is to the comb line at 1561.77 nm. The zoom-in spectrum with a span of 2 nm is shown as inset, and (b) the RF spectrum, showing the locked beat note between SG-DBR and comb at 11 GHz is recorded. The beat note generated between SG-DBR and adjacent comb line is also visible. Both the phase-locked and free-running cases are shown to illustrate the improved relative spectral coherence between the on-chip tunable laser and comb.

Fig. 7 (a) The measurement setup for the tuning resolution of our OFS, (b) Power spectra of an RF frequency signal at 25.7 GHz generated by beating between comb lines on a high-speed PD integrated in the packaged unit measured with different resolution bandwidth. The smaller peaks are of 60 Hz and its harmonics, appearing from the power source, (c) locked beat signal between reference comb line and the SG-DBR laser and its movement by 100 Hz, and (d) plot of change in the optical beat note with respect to change in the RF offset frequency.

Fig. 9 RF spectrum measured at the ESA of modulated SG-DBR laser beating with the comb output during dynamic wavelength switching of SG-DBR. Three peaks are seen, (1) the locked beat note is at 2.5 GHz, produced by beating between both SGDBR peaks and the corresponding comb lines, (2) The beat note generated between both SG-DBR peak and adjacent corresponding comb line is at 23.3 GHz, and (3) the beat note produced between comb lines is at 25.7 GHz.

Fig. 10 (a) A real-time oscilloscope trace of the external photodiode output of both wavelength component of SG-DBR laser during wavelength switching in order to measure the locking time of the OPLL system. Three periods are shown here which corresponds to the modulation frequency of front mirror, i.e. 800 kHz, (b) trace with a smaller span, showing the transition to phase-locking, and (c) trace with smallest span to show 2.5 GHz signal during phase-locking.